Application of the Fengyun 3 C GNSS occultation sounder for assessing the global ionospheric response to a magnetic storm event - Atmos. Meas. Tech
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Atmos. Meas. Tech., 12, 1483–1493, 2019
https://doi.org/10.5194/amt-12-1483-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 3.0 License.
Application of the Fengyun 3 C GNSS occultation sounder for
assessing the global ionospheric response to a magnetic storm event
Weihua Bai1,2,3 , Guojun Wang1,4 , Yueqiang Sun1,2,3 , Jiankui Shi1,3,4 , Guanglin Yang5 , Xiangguang Meng1,2 ,
Dongwei Wang1,2 , Qifei Du1,2 , Xianyi Wang1,2 , Junming Xia1,2 , Yuerong Cai1,2 , Congliang Liu1,2 , Wei Li1,2 ,
Chunjun Wu1,2 , Danyang Zhao1,2 , Di Wu1,2 , and Cheng Liu1,2
1 National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China
2 Beijing Key Laboratory of Space Environment, Beijing 100190, China
3 School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China
4 State Key Laboratory of Space Weather, Beijing 100190, China
5 National Satellite Meteorological Center, Chinese Meteorological Administration, Beijing 100081, China
Correspondence: Guojun Wang (gjwang@nssc.ac.cn)
Received: 7 September 2016 – Discussion started: 10 October 2016
Revised: 31 January 2019 – Accepted: 14 February 2019 – Published: 7 March 2019
Abstract. The rapid advancement of global navigation satel- of this storm event on the ionosphere. The analysis demon-
lite system (GNSS) occultation technology in recent years strates the reliability of the GNSS radio occultation sounding
has made it one of the most advanced space-based remote instrument GNOS aboard the FY3-C satellite and confirms
sensing technologies of the 21st century. GNSS radio oc- the utility of ionosphere products from GNOS for statistical
cultation has many advantages, including all-weather oper- and event-specific ionospheric physical analyses. Future FY3
ation, global coverage, high vertical resolution, high preci- series satellites and increasing numbers of Beidou navigation
sion, long-term stability, and self-calibration. Data products satellites will provide increasing GNOS occultation data on
from GNSS occultation sounding can greatly enhance iono- the ionosphere, which will contribute to ionosphere research
spheric observations and contribute to space weather mon- and forecasting applications.
itoring, forecasting, modeling, and research. In this study,
GNSS occultation sounder (GNOS) results from a radio oc-
cultation sounding payload aboard the Fengyun 3 C (FY3-C)
satellite were compared with ground-based ionosonde ob- 1 Introduction
servations. Correlation coefficients for peak electron den-
sity (NmF2) derived from GNOS Global Position System The Global navigation satellite system (GNSS) occultation
(GPS) and Beidou navigation system (BDS) products with technique uses occultation receivers mounted on low earth
ionosonde data were higher than 0.9, and standard devi- orbit (LEO) satellites to collect GNSS signals that are re-
ations were less than 20 %. Global ionospheric effects of fracted and delayed by the atmosphere and ionosphere. The
the strong magnetic storm event in March 2015 were ana- excess phase due to the atmosphere and ionosphere is deter-
lyzed using GNOS results supported by ionosonde observa- mined from measurements of the delayed carrier phase and
tions. The magnetic storm caused a significant disturbance in the precise positions and velocities of the LEO and GNSS
NmF2 level. Suppressed daytime and nighttime NmF2 lev- satellites. An inverse Abel transform method is used to derive
els indicated mainly negative storm conditions. In two lon- electron density of the ionosphere, the refraction index, tem-
gitude section zones of geomagnetic inclination between 40 perature, humidity, and atmospheric pressure data, as shown
and 80◦ , the results of average NmF2 observed by GNOS and in Fig. 1.
ground-based ionosondes showed the same basic trends dur- GNSS radio occultation technology makes global-scale
ing the geomagnetic storm and confirmed the negative effect measurements of the atmosphere and ionosphere possible.
It has the advantages of high precision, high vertical reso-
Published by Copernicus Publications on behalf of the European Geosciences Union.1484 W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS Figure 1. Sketch of GNSS radio occultation sounding technology (using China’s FY3-C satellite as an example). lution, long-term stability, global coverage, all-weather op- 2008). As well as using GNSS radio occultation data to ob- eration, and a relatively low cost, which can compensate tain ionospheric electron density profiles, new and innovative for some of the shortcomings of conventional atmospheric exploratory studies continue to emerge, for example, using and ionospheric sounding tools (e.g., Fu et al., 2007). The amplitude and signal-to-noise ratio data from radio occulta- global-scale data obtained have important scientific poten- tion to detect the Es layer (Hocke et al., 2001) and spread F tial for improving the accuracy of numerical weather predic- (Lu et al., 2011). tion, near-space environment monitoring research, global cli- The GNSS radio occultation sounder (GNOS) instrument mate change research, atmospheric modeling research, and (Fig. 2), developed by the National Space Science Center data assimilation. Radio occultation technology has signifi- (NSSC) of the Chinese Academy of Sciences (CAS), has cant scientific value and a broad array of potential practical accumulated large amounts of radio occultation data since applications in climatology, meteorology, ionospheric stud- it was launched into orbit aboard the Fengyun 3 C (FY3- ies, and geodesy. C) satellite on 23 September 2013. It was the first GNOS Radio occultation is extremely useful and important in instrument compatible with both Beidou navigation satellite ionosphere research, monitoring ionospheric anomalies, in- system (BDS) and Global Positioning System (GPS) technol- vestigating ionospheric scintillation, and forecasting space ogy. BDS is China’s global navigation satellite system, which weather. It also has a broad range of potential applications can provide coverage in the Asia-Pacific region, currently in communications, space operations, and national defense. with five geostationary orbit (GEO) satellites, five inclined Zhao et al. (2013) used ionosonde and radio occultation data geosynchronous orbit (IGSO) satellites, and five medium to analyze differences in the ionosphere in eastern and west- earth orbit (MEO) orbit satellites (BeiDou document, 2016). ern China, including the origins of ionospheric changes and FY3-C is in a sun-synchronous polar orbit at an altitude of latitudinal and longitudinal changes in structural layers of 836 km and inclination of 98.8◦ , and it has an orbital period the ionosphere. Liu et al. (2008, 2009, 2010, 2011) used of 101.5 min. The atmospheric refractivity profile of GNOS COSMIC radio occultation data to study seasonal changes has a precision better than 1 % in 5–25 km (Bai et al., 2014; in the electron density of the ionosphere, characteristics of Liao et al., 2016; Wang et al., 2015). Peak electron density in the low-latitude ionosphere, and the scale height of the iono- the ionosphere can be detected to within 20 % of ionosonde spheric peak. In addition, many researchers have used iono- measurements (Wang et al., 2015; Yang, 2015). spheric occultation data to search for ionospheric anoma- The purpose of this study is to explore applications of lies prior to earthquakes (Yang et al., 2008; Zhang et al., FY3-C GNOS ionospheric data products in space weather Atmos. Meas. Tech., 12, 1483–1493, 2019 www.atmos-meas-tech.net/12/1483/2019/
W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS 1485
cultation channels, which are able to simultaneously track
dual-frequency signals from six GPS satellites (including at-
mospheric and ionospheric occultation signals). It also has
four dual-frequency BDS occultation channels, which can si-
multaneously track four dual-frequency BDS satellite signals
(including atmospheric and ionospheric occultation signals).
The GNOS ionospheric occultation data mainly include dual-
system, dual-frequency carrier phase and SNR information,
with a sampling rate of 1 Hz. Since the primary mission is
atmospheric occultation sounding, this is given priority, so
that when there are no free channels, a new atmospheric oc-
cultation event occupies an ionospheric occultation channel.
Under these limited channel conditions, the actual number
Figure 2. The FY3-C GNSS occultation sounder GNOS. The of complete ionospheric occultation profiles that can be pro-
GNOS instrument consists of one electronic unit (EU), two fixed duced daily is around 220 from the GPS and around 130 from
occultation antennas (one forward occultation antenna, FOA, and the BDS. Since the FY3-C satellite is in a sun-synchronous
one aft occultation antenna, AOA), one position antenna (PA), and polar orbit, it passes ground stations at around 10:00 and
three radio frequency units (RFUs). 22:00 (LT); hence, occultation events are mainly concen-
trated in the two local time periods between 09:00 and 12:00
and 21:00 and 24:00.
research, specifically in the analysis of ionospheric NmF2
GNSS signals transmitted through the ionosphere and re-
patterns during the magnetic storm of March 2015, which
ceived by LEO satellites are bent and delayed by refraction
significantly affected the ionosphere (e.g., Astafyeva et al.,
in the ionosphere. Using dual-frequency phase observations,
2015; Nava et al., 2016). Results of the study lay the founda-
the corresponding total electron content (TEC) of the iono-
tion for the use of GNOS in space weather research, includ-
sphere can be obtained:
ing magnetic and ionospheric storms.
f12 f22
TEC = (L1 − L2 ) , (1)
2 Instrument performance and data validation C f12 − f22
The instrument GNOS aboard the FY3-C satellite (in Fig. 2) where L1 and L2 are the dual-frequency carrier phase ob-
is composed of one electronic unit (EU), two fixed oc- servations, C = 40.3082 m3 s−2 is a constant, and f1 and f2
cultation antennas (one forward occultation antenna, FOA, indicate the two frequencies. This type of dual-frequency
and one aft occultation antenna, AOA), one position an- TEC inversion method (Schreiner et al., 1999; Jin et al.,
tenna (PA), and three radio frequency units (RFUs). The 2014) eliminates clock errors (Jin et al., 2014) and uses an
electronic unit is located in the cabin. The forward and aft Abel transformation to obtain the ionospheric electron den-
occultation antennae are each electrically split into atmo- sity based on the assumption of local spherical symmetry
spheric and ionospheric components (Bai et al., 2014). The ionosphere:
ionospheric occultation antennas are single-unit, micro-strip,
dual-mode, and dual-frequency, and they can simultaneously LEO
Z
1 dTEC/dr
receive BDS dual-frequency (B1I 1561.098 MHz and B2I Ne (r0 ) = − q dr, (2)
1207.140 MHz) and GPS dual-frequency (L1 1575.420 MHz π r 2 − r02
r0
and L2 1227.600 MHz) ionospheric occultation signals. The
maximum gain of each antenna is 5 dB, and the half power where Ne (r) is electron density, r0 is the straight-line impact
beam width of the ionospheric occultation antenna is ±40◦ . distance, and r is the distance between the earth geometrical
The forward ionospheric occultation antenna is oriented nor- center and one point on the ray path.
mal to the +x axis of the satellite, i.e., the direction in which We evaluated the GNOS ionosphere electron density pro-
the satellite is moving. The aft ionospheric occultation an- files through statistical comparison with ionosonde data; for
tenna is oriented normal to the −x axis of the satellite. The detailed comparing results, we refer to Yang et al. (2018).
objective of this design is to make the beam of the iono- Electron density profile data were collected from GNOS,
spheric occultation antenna with maximum gain cover the covering a 365-day period between 1 October 2013 and
ionospheric occultation target region, in order to obtain a 30 September 2014, giving 62 381 occultation profiles from
high signal-to-noise-ratio (SNR) and a high-quality occulta- the GPS and 47 490 from the BDS. We collected iono-
tion signal. sphere observation data taken from ground ionosonde sta-
Within the power consumption limits aboard the FY3-C tions, which mainly comprised two parameters: maximum
satellite, GNOS is equipped with six dual-frequency GPS oc- electron density in the F2 layer of the ionosphere (NmF2)
www.atmos-meas-tech.net/12/1483/2019/ Atmos. Meas. Tech., 12, 1483–1493, 20191486 W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS
Global electron density profiles have been successfully
probed in several previous GNSS radio occultation missions,
including GPS/MET, CHAMP, and COSMIC (e.g., Garcia-
Fernandez et al., 2003; Habarulema et al., 2014). Using
ionosonde data for verification, their reported precisions are
NmF2 average bias 1 % and standard deviation 20 % for
GPS/MET (Hajj et al., 1998) and NmF2 average bias −1.7 %
and standard deviation 17.8 % for CHAMP (Jakowski et al.,
2002). The mean differences of the Nmf2 are insignificant
for COSMIC; relative variations of the peak differences are
determined in the range of 22 %–30 % for NmF2 and 10 %–
15 % for hmF2 (Limberger et al., 2015). In particular, even
during disturbed conditions, the relative difference of NmF2
and hmF2 agrees to within 21 % and 9 %, respectively, as
reported by Habarulema et al. (2016), who made the first
long-term comparison between RO and ionosonde NmF2 and
Figure 3. The distribution of ionosonde stations. The data of 54 hmF2 data over Grahamstown. GNOS GPS and BDS oc-
ionosonde stations can be obtained from GIRO. cultation NmF2 observations reported in this study have a
slightly higher average bias than the other systems, but their
good correlation coefficients and standard deviations demon-
and the altitude of the maximum electron density (hmF2). strate the overall reliability of the results.
Ionosonde data were obtained from 54 ionosonde stations
of the Global Ionospheric Radio Observatory (GIRO) of the
University of Massachusetts Lowell (UML) (as shown in
3 Analysis of GNOS results during the magnetic storm
Fig. 3). The criteria used for matching GNOS and ionosonde
of March 2015
data were a time interval within ±1 h and spherical distance
within 200 km. For every matching pair of data, the relative 3.1 Characteristics of the magnetic storm
error (R) in the GNOS NmF2 was calculated using Eq. (3):
NmF2GNOS − NmF2IONO Solar activity in 2015 was at a moderate level, and there
R= × 100 %, (3) were several large geomagnetic storm events. This study fo-
NmF2IONO
cuses on the magnetic storm event that occurred between
where the subscript IONO represents ionosonde. 11 and 31 March 2015, peaking at 05:22 UT on 17 March.
Figure 4 compares NmF2 and hmF2 measurements from Changes in magnetic indices during the storm are shown in
the GNOS GPS occultation and the ionosondes. Over the Fig. 6. The geomagnetic activity index Kp, which charac-
course of the year, a total of 745 matching pairs of data terizes global geomagnetic activity, was generally below 3
were collected. Linear regression of absolute NmF2 values before 17 March, then it increased significantly, with large
derived from each method (Fig. 4) gives a correlation coef- perturbations continuing until 26 March, after which it re-
ficient of 0.96, a statistical bias of 6.71 %, and standard de- turned to pre-storm levels. The Dst index, which is an index
viation of 18.03 %. The hmF2 correlation coefficient is 0.85, of geomagnetic activity used to assess the severity of mag-
the bias is 4.68 km, and standard deviation is 25.96 km. Fig- netic storms, was relatively stable before 17 March, hovering
ure 5 is similar to Fig. 4 but with GNOS BDS occultation around zero. The index suddenly increased between 04:00
profiles (354 BDS matching pairs) rather than GPS products; and 06:00 UT on 17 March, marking the initial phase of the
the NmF2 correlation coefficient of the fitted regression is magnetic storm, and then dropped rapidly during the main
0.96, the statistical bias is 8.31 %, and standard deviation is phase to a minimum of −223 nT at 23:00 UT. A rapid re-
17.24 %. The linear regression of hmF2 values derived from covery phase followed, from 23:00 UT on 17 March until
the GNOS BDS occultation and ionosonde data gives a cor- 18:00 UT on 18 March, then a slower recovery until around
relation coefficient of the fitted regression of 0.80, a statisti- 10:00 UT on 25 March, when it returned to pre-storm levels.
cal bias of 2.67 km, and standard deviation of 23.28 km. The The auroral electrojet (AE) index mainly reflects polar sub-
bias and standard deviation of the NmF2 derived from the storm intensity, with variations closely related to the quan-
GNOS BDS occultation and GPS occultation are consistent tity of particles injected into polar regions. The AL and AU
and comparable. However, the bias and standard deviation of indices reflect westward and eastward electrojet conditions,
the NmF2 from BDS are slightly larger; this could be caused and the AE index is the absolute difference between them.
by the larger position errors of the BDS satellites, especially During the main phase of the magnetic storm, the magni-
for the GEO satellites, and the different distribution of the tude of AE index perturbations reached around 1500, with
occultation events. frequent lower magnitude perturbations during the recovery
Atmos. Meas. Tech., 12, 1483–1493, 2019 www.atmos-meas-tech.net/12/1483/2019/W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS 1487
Figure 4. Comparison of NmF2 measurements from GNOS GPS occultation and ionosondes. Panel (a) shows a linear regression of the
absolute NmF2 values measured using the two methods. The black line is y = x, the red line is the fitted regression, corr. coef. is the
correlation coefficient, bias is the statistical bias, and SD is the standard deviation. Panel (b) shows a linear regression of the absolute hmF2
values measured using the two methods (source: Yang et al., 2018).
Figure 5. Same as Fig. 4, but for GNOS Beidou navigation system (BDS). Comparison of NmF2 and hmF2 measurements from GNOS BDS
occultation and ionosondes. Panel (a) shows a NmF2 linear regression fit between the two sets of measurements; panel (b) shows a hmF2
linear regression fit between the two sets of measurements.
phase. Overall, this magnetic storm event caused severe geo- covery phase). Around 350 ionosphere occultation profiles
magnetic disturbances on a global scale. As plasma in the were recorded daily (220 GPS + 130 BDS). Although the
ionosphere is controlled by the earth’s magnetic field, the quantity of data is limited due to the rather high inclina-
global ionosphere was also affected by the magnetic storm. tion polar orbit of FY3-C, recorded occultation events are
distributed across the globe, with a relative concentration
3.2 Global GNOS results during the magnetic storm at higher latitudes. Figure 7a shows that, before the mag-
netic storm, the highest daytime NmF2 values were mostly
Figure 7 shows global daytime (07:00–17:00 LT; Fig. 7a) and distributed around the magnetic equator, with relatively low
nighttime (19:00–05:00 LT; Fig. 7b) occultation event distri- electron densities at high magnetic latitudes. During the main
butions for 14 March 2015 (pre-storm calm), 17 March (main phase of the storm, NmF2 perturbations were significant,
phase), 18 March (rapid recovery phase), and 22 March (re-
www.atmos-meas-tech.net/12/1483/2019/ Atmos. Meas. Tech., 12, 1483–1493, 20191488 W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS
Figure 6. Variations of geomagnetic indices (Kp, ap, Dst, AE, AU, AL, and AO) during the magnetic storm of March 2015 (data from the
International Service of Geomagnetic Indices (ISGI) website: http://isgi.latmos.ipsl.fr/ (last access: 10 September 2016)).
with especially large increases in the South Atlantic region. Meridian Project stations, is shown in Fig. 8. According to
In the southeastern Pacific, NmF2 decreased during the main the distribution of these stations, we classify these stations
storm phase and increased during the recovery phase, re- into two zones. Zone 1, including eight SWPC stations, is
turning to pre-storm levels by 22 March. Nighttime GNOS within magnetic inclinations of 40–80◦ and longitudes of
soundings (Fig. 7b) show suppressed NmF2 values in East −20–40◦ , mainly in the Europe region. Zone 2, including
Asia and Australia during the main phase and start of the five SWPC stations and two China Meridian Project stations,
recovery phase of the storm. NmF2 values had returned to is located within magnetic inclinations of 40–80◦ and longi-
pre-storm levels by 22 March. These results demonstrate the tudes of 100–160◦ , mainly in the East Asia region.
capability of FY3-C GNOS to characterize the global NmF2 Figures 9 and 10 plot NmF2 variations at each station in
response to magnetic storm events. zone 1 and zone 2, respectively; blue lines denote the SWPC
stations, and the red lines denote the China Meridian Project
3.3 Statistical analysis of GNOS ionosphere products stations. Perhaps the most outstanding feature is a surge in
and ionosonde observations during the magnetic NmF2 values at individual stations on 17 March, during the
storm period main phase of the magnetic storm (e.g., around 12:00 LT at
Moscow), with a decrease after 16:00 at many stations. Sev-
The daily GNOS ionosphere profiles are relatively sparse and eral stations show a significant decrease in NmF2 measure-
unevenly distributed around the globe. Therefore, it is diffi- ments during the beginning of the recovery phase, on 18 and
cult to quantitatively analyze the response of a specific loca- 19 March, except for a few European stations in zone 1 (e.g.,
tion during a magnetic storm event. However, as most occul- Rome). After 19 March, NmF2 patterns become more com-
tation events are distributed at midlatitudes and high latitudes plex, with some stations (e.g., Mohe) showing values below
(magnetic inclinations of 40–80◦ in both hemispheres), it is pre-storm levels and others (e.g., Okinawa) about the same
possible to analyze changes in average NmF2 values in spe- as pre-storm levels. Overall, ionospheric perturbations dur-
cific zones during a magnetic storm. ing the magnetic storm were mainly negative.
We also compare GNOS measurements’ statistical analy- Figure 11a and b compare error bars of NmF2 values from
sis with ionosonde data from the US Space Weather Predic- the ionosonde stations with those from the GNOS occultation
tion Center (SWPC) worldwide stations and China Merid- products in two zones, respectively, for different time peri-
ian Project stations. As there are very few ionosonde sta- ods in the magnetic storm event, including 9:00–12:00 and
tions located at magnetic inclinations of 40–80◦ in the South- 21:00–24:00 LT (most of the GNOS occultation events were
ern Hemisphere, we focused on NmF2 data from the 15 concentrated around these times). Figure 11a compares error
ground stations in the Northern Hemisphere in the period 14– bars in zone 1, mainly in the Europe region, and Fig. 11b
22 March 2015. The geographic distribution of these 15 sta- represents the results in zone 2, mainly in the East Asia re-
tions, including 13 SWPC ionosonde stations and two China gion. In these plots, NmF2 trends are very similar for the two
Atmos. Meas. Tech., 12, 1483–1493, 2019 www.atmos-meas-tech.net/12/1483/2019/W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS 1489 Figure 7. (a) NmF2 values detected by GNOS in all daytime (07:00–17:00 LT) occultation events on typical representative days (14 March calm, 17 March magnetic storm main phase, 18 March rapid recovery phase, and 22 March steady recovery phase). Squares represent the geographic location of GNOS GPS occultation events. Diamonds represent the geographic location of GNOS BDS occultation events. The color of each square or diamond represents NmF2 magnitude for that occultation event. Black contours represent geomagnetic inclination isolines. (b) Same as (a), but for the NmF2 values detected by GNOS in all nighttime (19:00–5:00 LT) occultation events. observation methods, with the negative storm effects of the the spatial observations between the two measurement tech- recovery phase quite clear. However, GNOS measurements, niques; i.e., GNOS can observe randomly, whereas the loca- especially in the 21:00–24:00 LT time block, show larger tions of the ionosondes are fixed on the continents. Nighttime perturbations than those of the ionosonde stations. These (21:00–24:00 LT) average NmF2 levels were much lower in discrepancies may be due to the significant differences of the main phase (17 March) and at the start of the recov- www.atmos-meas-tech.net/12/1483/2019/ Atmos. Meas. Tech., 12, 1483–1493, 2019
1490 W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS
Figure 8. Distribution of ionosonde stations located in two zones.
Zone 1, including eight SWPC stations, is within magnetic incli-
nations of 40–80◦ and longitudes of −20–40◦ , mainly in the Eu-
rope region. Zone 2, including five SWPC stations and two China
Meridian Project stations, is located within magnetic inclinations
of 40–80◦ and longitudes of 100–160◦ , mainly in the East Asia re-
gion. Black squares represent SWPC stations, and red stars repre-
sent China Meridian Project stations.
Figure 10. Variations in NmF2 at five SWPC stations and two China
Meridian Project ground ionosonde stations in zone 2 during the
magnetic storm of March 2015. The blue lines represent the SWPC
station, and the red lines denote the China Meridian Project station.
Fig. 11a, daytime average NmF2 is shown to have reached
a minimum on 18 March from GNOS data, but it reached a
minimum on 20 March and then slowly recovered.
Overall, the magnetic storm caused significant distur-
bances in average NmF2 in both zone 1 and zone 2. Both
daytime and nighttime NmF2 values showed mainly nega-
tive storm characteristics; NmF2 decreased significantly dur-
ing the main phase of the storm and increased tardily during
the recovery phase.
4 Discussion
The GNSS radio occultation allows monitoring of electron
density in the ionosphere at a global scale. It has the ad-
vantages of high accuracy, good vertical resolution, global
coverage, and all-weather capability. However, an important
Figure 9. Variations in NmF2 at eight ground ionosonde stations
located in zone 1 during the magnetic storm of March 2015.
constraint in applications of the occultation electron den-
Ionosonde data were obtained from SWPC. sity products is the assumption of a symmetrical ionosphere
in the inverse Abel transformation calculations (Garcia-
Fernandez et al., 2003; Yue et al., 2010, 2011). In reality, it is
ery phase (18–20 March) than before the storm, reaching a very difficult to guarantee symmetrical distribution of elec-
minimum on 18 March. Then, the average NmF2 levels rose tron density in the ionosphere, especially near anomalies at
slowly during the whole recovery phase. Daytime average the magnetic equator and high latitudes. Nevertheless, com-
NmF2 in zone 2 reached a maximum during the main phase parison of the GNOS probe results with ionosonde measure-
of the magnetic storm on 17 March, falling to a minimum ments provided a correlation coefficient of 0.95 and standard
on 19 March, after which it slowly increased. In zone 1 in deviation less than 20 %, which are basically coincident with
Atmos. Meas. Tech., 12, 1483–1493, 2019 www.atmos-meas-tech.net/12/1483/2019/W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS 1491
using global and regional electron content. Here, we ana-
lyzed the effect of the March 2015 magnetic storm on the
global ionosphere using data from FY3-C GNOS and from
ground-based ionosonde stations. In terms of spatial distri-
bution, Fig. 7a shows that the daytime NmF2 values in the
South Atlantic region were elevated during the main phase
of the storm compared with ones on 14 March before the
storm. This increase in NmF2 during the main phase also
occurred in the northern Africa region, which is consistent
with the ionospheric positive storm effect reported in Fig. 3
of Astafyeva et al. (2015) and in Fig. 4 of Nava et al. (2016).
In the southeastern Pacific, the daytime NmF2 values seemed
to decrease a little during the main phase of the storm, and
then they increased in the recovery phase. This is little differ-
ent from the results of Astafyeva et al. (2015), who presented
a significant positive storm effect during the main phase and a
negative storm at the beginning of the recovery phase at low
latitudes in the eastern Pacific sector. This difference may
be due to the fact that their focused latitude region is lower
than ours. Figure 7b shows that the nighttime NmF2 values
around the East Asia and Australia sector were mainly sup-
pressed during the main phase and at the beginning of the re-
covery phase then returned to pre-storm levels by 22 March.
These results are consistent with those reported by Nava et
al. (2016).
It was not possible to perform long-term continuous com-
parison between ionosonde and GNOS data, as the number
of occultation events recorded by GNOS was insufficient.
Hence, the results from two zones were averaged to enable
a quantitative comparison. In addition, the statistical signifi-
cance of this method is based on the assumption that the ef-
fects of the magnetic storm are basically consistent through-
out the ionosphere in each zone. Figure 11a and b display
the statistical comparison of the averaged data, showing sim-
ilar general trends in measurements from the ionosonde sta-
tions and GNOS during the storm. The comparison further
confirmed the nature of the magnetic storm and the negative
Figure 11. (a) Comparison of error bars of NmF2 values from the effects of the storm on the ionosphere. The negative storm
eight ionosonde stations and GNOS in zone 1 in the Northern Hemi- in ionosphere parameters is usually caused by composition
sphere during the magnetic storm of March 2015. (b) Same as (a), changes. During the storm, the high-latitude atmosphere will
but for the comparison of error bars of NmF2 values from the seven be heated heavily, which results in upwelling of the molec-
ionosonde stations and GNOS in zone 2. ular rich neutral gas into the upper thermosphere and pro-
vokes the neutral gas circulation from the pole to the Equa-
tor (e.g., Fuller-Rowell et al., 1996). Furthermore, the ion
previous studies (e.g., Habarulema et al., 2016). Therefore, density loss is proportional to the molecular concentration;
in the majority of cases, GNOS results may be considered to thus the negative storm will occur over the regions where the
be reliable and reasonable. molecular mass increases. In Fig. 9, a few stations (such as
The March 2015 magnetic storm was up to now the Gibilmanna) are shown to have observed a sustained day-
strongest in the recent solar cycle and severely disturbed time NmF2 enhancement on 18 March. The exact reasons
the global ionosphere, which has been extensively inves- for this enhancement are not clear. The major contributors
tigated. For instance, Astafyeva et al. (2015) have shown could include neutral composition changes (Crowley et al.,
multi-instrumental results on the global ionospheric response 2006), thermospheric winds (Balan et al., 2011), disturbance
to the storm. Nava et al. (2016) have also presented the of the dynamo electric field (Richmond and Lu, 2000), or
effects of the storm over the midlatitude and low-latitude some combination of these factors. In fact, the way in which
ionosphere in Asian, African, American, and Pacific sectors the global ionosphere responds to magnetic storms is ex-
www.atmos-meas-tech.net/12/1483/2019/ Atmos. Meas. Tech., 12, 1483–1493, 20191492 W. Bai et al.: Assessing ionospheric response to magnetic storm by FY3-C GNOS
tremely complicated. From Fig. 7a, we can see that this par- Data availability. The data of China Meridian Project are available
ticular magnetic storm caused varying responses in the iono- at https://data.meridianproject.ac.cn/ (last access: 10 August 2016).
sphere at different times and locations. Many factors influ- The data of FY3-C GNOS are available at http://satellite.nsmc.org.
ence the ionosphere, such as neutral composition changes, cn/portalsite/Data/DataView.aspx (last access: 10 August 2016).
electric fields, and neutral winds. For a specific magnetic The data of SWPC are available at ftp://ftp.swpc.noaa.gov/pub/lists/
iono_month/ (last access: 10 August 2016).
storm, corresponding ionospheric perturbations also depend
on the season, solar activity, local time of magnetic storm
occurrence, and the latitude and longitude. Therefore, iono-
Competing interests. The authors declare that they have no conflict
spheric storms are extremely complex; no two storms are pre-
of interest.
cisely alike, and the mechanisms that generate them also vary
(Balan et al., 1990; Fuller-Rowell et al., 1996; Danilov et al.,
2001; Mendilo et al., 2006). In addition, the type and form Acknowledgements. We thank UML GIRO and SWPC for provid-
of a magnetic storm also make a difference in the way that ing the ionosonde data. We also acknowledge the use of data from
it affects the ionosphere (Zhang et al., 1995). Future anal- the Chinese Meridian Project. This research was supported jointly
ysis incorporating assimilation with other data sources and by the National Science Fund of China (41505030, 41405039,
models may allow the precise mechanisms responsible for 41405040, 41574150, 41606206, 41674145, and 41474137) and
ionospheric effects of the March 2015 magnetic storm to be the Scientific Research Project of the Chinese Academy of Sciences
determined. (YZ201129), as well as the Specialized Research Fund for State
Key Laboratories of China.
5 Conclusions Edited by: Andreas Richter
Reviewed by: Chris Watson and one anonymous referee
The comparison of ionosonde data and FY3-C GNOS radio
occultation products presented in this study shows that, in the
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